Method and device for synchronizing the global time of a plurality of TTCAN buses and a corresponding bus system

ABSTRACT

A method, a device, and a bus system for synchronizing at least two TTCAN buses having at least one bus user, a global time being determined in each TTCAN bus, and the deviations in the global times of the TTCAN buses being determined from the global times, the TTCAN buses being connected via at least one user and the deviations in the individual global times being transmitted to at least one bus user, and the global times of the TTCAN buses connected via at least one user being adjusted to one another as a function of the deviations in the global times, so that the TTCAN buses are synchronized with respect to the global times.

FIELD OF THE INVENTION

The present invention relates to a method and a device for synchronizingthe global time in at least two TTCAN buses, as well as to acorresponding bus system.

BACKGROUND INFORMATION

In recent years, there has been a drastic increase in the networking ofcontrol units, sensors, and actuators by a communication system, i.e. bya bus system, in the manufacturing of modem motor vehicles and machineconstruction, in particular in both the machine tool sector andautomation. Synergistic effects may be achieved here due to thedistribution of functions among a plurality of control units. These areknown as distributed systems. Communication between different stationsis increasingly being accomplished via at least one bus or at least onebus system. Communication traffic on the bus system and the access andreception mechanisms, as well as error handling, are regulated by aprotocol.

The CAN (controller area network) protocol is well established in theautomotive sector. This is an event-driven protocol, i.e. protocolactivities such as sending a message are initiated by events which havetheir origin outside the communications system. Unique access to thecommunication system or bus system, is triggered via priority-based bitarbitration. A prerequisite for this is that a priority is assigned toeach message. The CAN protocol is very flexible. It is, thus, readilypossible to add additional nodes and messages as long as there are stillfree priorities (message identifiers). The collection of all messages tobe sent in the network, together with their priorities and transmitternodes, plus possible reception nodes, are stored in a list known as thecommunication matrix.

An alternative approach to event-driven, spontaneous communication isthe purely time-triggered approach. All communication activities on thebus are strictly periodic. Protocol activities such as sending a messageare triggered only by the advance of a time valid for the entire bussystem. Access to the medium is based on the apportionment of timeperiods during which a transmitter has an exclusive transmission right.The protocol is comparatively inflexible; new nodes may only be added ifthe corresponding time ranges have already been freed up in advance.This circumstance requires that the order of messages already bedetermined before starting operation. Thus, a timetable is drawn upwhich has to meet the requirements of the message with respect to rateof repetition, redundancy, deadlines, etc. One speaks of a so-called busschedule. The positioning of messages within the transmission periodsmust be coordinated with the applications which produce the messagecontent to minimize the latency between the application and thetransmission time. If this matching does not take place, the advantageof the time-controlled transmission (minimal latent jitter when sendingthe message on the bus) is destroyed. Thus, high demands are made on theplanning tools. TTP/C is such a bus system.

The requirements outlined above for a time-triggered communication andthe requirements for a certain measure of flexibility are met by themethod of time-triggered CAN known as TTCAN (time-triggered controllerarea network), which is described in German Published Patent ApplicationNo. 100 00 302, German Published Patent Application No. 100 00 303,German Published Patent Application No. 100 00 304 and German PublishedPatent Application No. 100 00 305, as well as in ISO Standard 11898-4(currently in the form of a draft). TTCAN meets these requirements byestablishing the communication cycle (basic cycle) in so-calledexclusive time windows for periodic messages of certain communicationusers, and in so-called arbitrating time windows for spontaneousmessages of a plurality of communication users. TTCAN is essentiallybased on a time-triggered, periodic communication, which is clocked by auser or node, which gives the operating time and is known as the timemaster, with the help of a time reference message, or, for short,reference message. The period until the next reference message is knownas the basic cycle and is subdivided into a predefinable number of timewindows. In this context, a distinction is made between the local times,i.e. local timers of the individual users, i.e. nodes, and the time ofthe time master as the global time of its timer. Additional principlesand definitions based on TTCAN are given in the ISO Draft 11898-4 or thementioned related art and are thus assumed to be known and will not beexplicitly described again.

Thus, there are numerous real time bus systems for networking controlunits in automation, in motor vehicles or elsewhere, including theaforementioned CAN, TTP/C or Byteflight, as well as the TTCAN justmentioned. CAN, TTCAN and Byteflight are single-channel bus systems,which means that redundancy may be achieved by duplicating thecorresponding system. TTP/C is intrinsically a two-channel system, i.e.the redundancy is always built in. Many bus systems offer a time basesynchronized with the bus as a service. In the bus systems designed fromthe start as two-channel or multichannel solutions, the synchronizationper design is typically forced in that a node or user mustsimultaneously transmit on the two buses. This has advantages (e.g.synchronization is always ensured), but it also has a number ofdisadvantages, such as the fact that not every bus may be operated byitself, the time patterns on the two buses may only differ to a verylimited extent, and the modularity of the two or more bus systems isweakened due to the coupling which is provided by design.

As explained, it is obvious that the related art is incapable ofyielding optimum results in every respect. This situation is to beimproved in the following.

In the case of buses or bus systems designed as single-channel systems,synchronization is performed explicitly if needed. In the following, aTTCAN network is assumed to be a bus system, or it is assumed that thereare a plurality of TTCAN buses or bus systems, and that they arecoupled, but this is only to be understood as restrictive with regard tothe object of the present invention to be explained later, inasmuch asthe properties of the TTCAN are a prerequisite or necessity forrepresenting the object according to the present invention.

SUMMARY OF THE INVENTION

The present invention relates to a method, a device, and a bus systemfor synchronizing at least two TTCAN buses having at least one bus user,a global time being determined in each TTCAN bus, and the deviations inthe global times of the TTCAN buses being determined from the globaltimes, these buses being interconnected via at least one user, and thedeviations in the individual global times being transmitted to at leastone bus user, and the global times of the TTCAN buses connected via atleast one user being adjusted to one another as a function of thedeviations in the global times, so that the buses are synchronized withrespect to the global times.

The global times of the TTCAN buses connected by at least one user areadvantageously adjusted by adjusting the phase of the global time.

The global times of the TTCAN buses connected by at least one user areexpediently adjusted by adjusting the frequency of the global time.

In an advantageous embodiment, the phase of the global time is adjustedas a function of the value of at least one predefined bit.

It is also advantageous that the at least one predefined bit correspondsto the discontinuity bit of the TTCAN.

A smallest time unit (NTU) in the at least two TTCAN buses to besynchronized is advantageously determined for adjusting the frequency tothe global time, and the ratio of these smallest time units isdetermined and the resulting real ratio is compared to a predefinedratio, at least one smallest time unit being adjusted so that thepredefined ratio is achieved.

In addition, it is advantageous that the predefined ratio corresponds toan integer, in particular a multiple or submultiple of the number two.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the coupling of two TTCAN bus systems by a user whichfunctions as a gateway user.

FIG. 2 shows the coupling of three TTCAN bus systems by coupled pairs.

FIG. 3 shows the coupling of four TTCAN bus systems by various users toillustrate scalable tolerance.

FIG. 4 shows a flow chart of frequency adjustment between two TTCANbuses or TTCAN bus systems.

FIG. 5 shows a flow chart to illustrate the phase adjustment of twoTTCAN buses or TTCAN bus systems.

DETAILED DESCRIPTION

The present invention describes general possibilities for how afault-tolerant bus system or network may be created from a combinationof several TTCAN buses. This is particularly advantageous in conjunctionwith the mechanisms with regard to synchronization of the global time ofa plurality of TTCAN buses and/or synchronization of the cycle time of aplurality of TTCAN buses, so that mutual synchronization of all thesebuses in the entire bus system or network may be accomplished.

FIG. 1 shows a bus system or network made up of a plurality of TTCANbuses or TTCAN bus systems, in this case two. A first bus is representedby 103B1, and a second bus is represented by 103B2. Two users 101 and102 are connected to the first bus 103B1. A user 105 is connected tosecond bus 103B2. User 100 is connected to both buses 103B1 and 103B2and functions as a connection user or gateway computer, i.e. gatewayuser, or also a gateway controller which has access to the two buses. Acoupled pair of TTCAN buses (103B1 and 103B2 here) is thus defined as acombination of two TTCAN buses, such that there is at least one gatewayuser having access to both buses. The connection of the individual usersto the particular bus is accomplished via a corresponding interfaceelement, e.g. interface element 110B1 in the case of user 101. Likewise,user 100 is connected as a gateway user to bus 103B1 via an interfaceelement 104B1, and to bus 103B2 via an interface element 104B2. As analternative, one interface element having two terminals may also beprovided for connection to bus 103B1 and to bus 103B2, in contrast withtwo interface elements 104B1 and 104B2.

Timers 111 and 106 having an internal clock source or time source 107and 112, in particular a quartz crystal or an oscillator, especially aVCO (voltage controlled oscillator), are also shown in users 100 and101. In addition, time detection modules, in particular counters 108 and113, are also contained in timers 106 and 111, respectively.

Control functions in the particular user, in particular for input/outputof data to the bus system, for transfer of time information from thetimer, and for synchronization of the buses and bus users, as well asother methods and method steps, etc., in particular those according tothe present invention, may be performed by modules 109 and/or 114 asprocessing modules, in particular a microcomputer or microprocessor orcontroller. Parts of this functionality or the entire functionality mayalso be provided directly in the particular interface module.

In this context, one user may also be preselected as a time referencegenerator as defined by TTCAN, in particular per bus system. This user,the time reference generator in the form of a time master, thereforespecifies the basic cycle as described in the related art. In the sameway, it is possible for the gateway user to function as a time referencegenerator, i.e. as a time master for the two bus systems. The timegenerator of the corresponding reference user, i.e. the time master ofthe particular TTCAN system by which the local time of this time masteris determined, is thus considered to be the reference time generator,i.e. it specifies the reference time for the corresponding bus system,i.e. 103B1 and/or 103B2. In other words, the local time generator, e.g.106 and/or 111 of the specified reference user as the time master, isthus considered a global time generator of the corresponding bus or bussystem 103B1 and/or 103B2 and specifies the global time of thecorresponding bus.

Therefore, FIG. 1 shows a coupled pair of TTCAN buses having a gatewayuser or gateway node. For more precise representation of this couplingaccording to the present invention, the following description accordingto the present invention is used:

At least two TTCAN buses B1, B2 are coupled when there is a seriesPi=(BXi, BYi), where i=1 to n, and there are n elements N having thefollowing properties:

BXi, BYi are TTCAN buses for all i,

For an i, BXi and BYi form a coupled pair of TTCAN buses,

BX(i+1) is BYi (for i=1 to n−1),

BX1 is bus B1 and BYn is bus B2.

In other words, two TTCAN buses B1 and B2 are coupled when they areconnected by some path of coupled pairs that is as complex. A system ofat least two TTCAN buses is referred to here as a fault-tolerant TTCANbus system, if two of the buses are coupled (in the sense describedabove). Therefore, all system architectures using a fault-tolerant TTCANbus system or network are detectable.

Additional examples are shown in FIGS. 2 and 3. FIG. 2 shows three TTCANbuses 203B1, 203B2, and 203B3 as well as bus users 200, 201, 204, and205. Buses 203B1 and 203B2 are linked together by user 200, and buses203B2 and 203B3 are linked together by user 201 in the same manner.Thus, in the sense according to the present invention, the coupling ofbus systems 203B1 and 203B2 and 203B2 and 203B3 as coupled pairs byusers 200 and 201 also allows bus systems 203B1 and 203B3 to beconnected in a fault-tolerant manner according to the definition givenabove, in particular with respect to synchronization. In contrast toconventionally redundant systems, i.e. two buses in the entire bussystem, where each node or user is connected in a redundant manner toevery other bus, i.e. each user has a connection to each bus, the systemarchitectures proposed here allow a scalable fault tolerance and amixture of fault-tolerant and non-fault-tolerant systems due to the useof coupled pairs.

This is explained once more, using FIG. 3 as an example. Four buses303B1, 303B2, 303B3, and 303B4 are shown in it. In addition, bus users301, 302, 300, 304, and 305 are also shown. Buses 303B1 and 303B2 arelinked together by user 300, and buses 303B3 and 303B4 are linkedtogether by user 301. At the same time, three buses 303B1, 303B2, and303B3 are interconnected by a user 302 so that, on one hand, a mixtureof fault-tolerant and non-fault-tolerant systems is made possible, and,on the other hand, the desired fault tolerance in the system isscalable, i.e. it may be represented in various degrees of redundancy(single, double, multiple redundancy). Therefore, it is possible tointroduce higher degrees of redundancy into a system, without couplingsystems that should remain uncoupled. Consequently, a reduction incommon-mode faults, i.e. common-mode faults of the bus system or the bussystems, is likewise possible.

In combination with the synchronization mechanisms in TTCAN, it is thuspossible to create a uniform synchronized communication system thatpermits all conceivable degrees of fault tolerance in the simplestmanner. Synchronization is described in greater detail below withrespect to the global time as well as the cycle times of individualusers or bus systems.

A general method of how two or more TTCAN buses, in particular level 2buses (see ISO Draft), are able to synchronize their global times withone another will be described first. This method may be used bydedicated hardware, by the applications running on the correspondinghost, or by a special software layer.

The method of synchronization, including possible variations in thesequence, is described below. A pair of buses is referred to here asbeing directly synchronizable, if there is at least one gateway computerhaving access to both buses, as described above. It is also a generalprerequisite that, between two buses to be synchronized, there is achain of gateways connecting the two buses via directly synchronizablepairs, i.e. the coupled pairs mentioned above. The synchronization layer(hardware or software) performing the synchronization is referred tobelow as synchronization layer SL. In doing so, the SL need notnecessarily be present on each node, as also indicated by the followingdescription.

In one embodiment, the gateway user or gateway computer is the timemaster in at least one of the two buses to be synchronized, in order toadjust the time in the bus in which it is time master. Optionally, as anadditional embodiment, a message may be transmitted from the gatewayuser to the time master to perform the corresponding time adjustment forthe synchronization. Then, it is not necessary for the gateway user toalso be time master of at least one of the bus systems to besynchronized.

On each TTCAN bus, the initialization procedure specified for this busby ISO 11898-4 and the related art for the TTCAN is run through forinitialization. As a result, one obtains two or more TTCAN buses, whichoperate independently of each other and have different global times anddifferent current time masters. In an optional variant, the systemdesign allows one to ensure that the same node or user becomes the timemaster on both buses or on a plurality of buses.

Then, on the basis of FIGS. 4 and 5, frequency adjustment and phaseadjustment are described, whereby the frequency and the phase of thebuses are adjustable independently of one another; in an advantageousembodiment, first the frequency is adjusted because, as long as thefrequency is incorrect, i.e. deviates, the phase will be changedcontinuously.

A) Frequency Adjustment between Two Buses

In the TTCAN, the rate of the global time, i.e. the length of time unitNTU, is determined by the timer frequency of the time master, i.e. inparticular the oscillator frequency or quartz crystal frequency and itsTUR (time unit ratio) value. In this context, NTU (network time unit)refers to the time units of the global time of the particular bus, andTUR refers to the ratio between the length of an NTU and the length of aspecific basic time unit, e.g. the local timer period, i.e. inparticular the local oscillator period as described in ISO Draft11898-4. Synchronization layer SL must ensure that the NTU's on thedifferent buses have the intended ratios to one another. The SL may berepresented in hardware or software by processing units 109 and/or 114,etc., but, as mentioned already, the SL need not necessarily be inherentin all users.

In principle, different procedures are conceivable. One bus may becoordinated with the other by selecting the frequency of one bus, i.e.the rate of the global time of the one bus as a setpoint and determiningthe difference in rate with respect to it, i.e. frequency, of the of thebus; the at least one additional bus being coordinated with the other orthe at least two buses approaching each other with respect to the rateof the global time, i.e. in particular the frequency of the timer of theparticular time masters. In addition, the adjustment may be performed inone step or gradually. The particular strategy depends on the SL, i.e.the synchronization layer, and the requirements of the particularapplications. All methods have the following system in common:

-   -   The SL determines the correction to be performed for a bus and        transmits it to the current time master. The correction may be,        for example, the ratio of the current TUR value and the new TUR        value to be set. It is particularly advantageous if the SL        determines the correction on the time master, so that the        correction value may be the new TUR value directly, for example.    -   The SL determines the new TUR value in the time master and uses        it, starting with the following cycle.    -   All the other nodes of this bus then follow the time master over        the TTCAN synchronization.

To this end, the global time of the first bus, e.g. B1, is determined,e.g. 103B1, in block 400 of FIG. 4. This is determined at a time T1 inblock 401, i.e. captured, so that a first capture value of the globaltime of bus B1 is obtained in block 401. At time T1, a value of theglobal time of bus B2 determined in block 404, e.g. 103B2, is detected,i.e. captured, in block 405. At subsequent time T2, the first capturevalue is advanced from block 401 and block 405 to block 402 and block406, respectively, and a new, second capture value is detected withrespect to the global time of the particular bus in block 401 or 405.From these two capture values in block 401 and block 402 and block 405and block 406, the rate of the global time or the clock rate of bussesB1 and B2, respectively, is determined by forming the difference inblock 403 and block 407, respectively. Then, by forming the differencein block 409, a correction value is obtained from these values for therate of the global time of the particular bus, this correction valuerepresenting the difference in the rate of the global times of the busesin question, i.e. the difference in the particular clock rate, i.e. thetimer rate. As mentioned above, this may also occur or be carried out,e.g. using the specific TUR value. The rest of the method of the presentinvention then proceeds with the aid of the value, which was determinedin block 409 and relates to the frequency adjustment in block 401.

The frequency adjustment is to be illustrated again, using an example.There are two buses, B1 and B2. The synchronization strategy is that B2must adjust the length of the NTU to the length valid on B1. In thesimplest case, it is assumed that both are nominally the same length.The application (SL) of the current time master of B2 is assumed to havedirect access to bus B1, and the same timer, i.e. the same clock sourceor time source, i.e. the same oscillator or quartz crystal, is to beused for B1 and B2. Then, the NTU's of the two buses B1 and B2 areexactly the same length when the TUR value of this node or user withrespect to bus B1 is equal to the TUR value of this node and/or userwith respect to bus B2. Therefore, the SL must use the TUR value withrespect to B1 as the TUR value with respect to B2.

In principle, the situation may be handled in the same way, if the NTU'sof the two buses have any desired ratio to one another. This is theneasily implementable in hardware in an advantageous manner, if the ratioor its reciprocal is an integer and, in a particularly advantageousembodiment, a power of two.

If the same timer, i.e. the same time source, is not used for bothbuses, then there is the possibility of measuring the difference in theglobal times of the two buses twice or more often, i.e. periodically insuccession, and calculating a correction factor from the comparisonbetween the observed change in the difference and the nominal change inthe difference. This possibility also exists if the current time masterdoes not have access to the two buses itself. As an alternative, the SLmay measure the length of a basic cycle on a bus in units of the otherbus and then determine the correction value from it.

B) Phase Adjustment

For the phase adjustment, the SL measures the phase difference betweentwo global times on two buses and determines the jump, i.e. correctionvalue, to be set for each of the two buses. This may be accomplished inan advantageous manner with the aid of the stopwatch register of theTTCAN.

The SL then transmits the jump or correction value to be set to the twocurrent time masters of the two buses. A time master that must set ajump sets a predefined bit, specifically the discontinuity bit in thecase of TTCAN, in the next reference message and shifts its global timeby the corresponding amount. In this manner, the time reference message,i.e. the reference message, of the corresponding time master is thensent at the adjusted time. After this (these) reference message(s) aresuccessfully transmitted on at least one of the two buses, the at leasttwo buses are synchronized with one another, if necessary.

FIG. 5 shows this phase adjustment. The global times of two buses areascertained in blocks 500 and 504 and acquired, i.e. captured, in block501 and block 505. In this context, capturing preferably occurs at thesame time T1. The two computer values are now sent directly to asubtractor in block 509 where the phase difference, i.e. the differencein the clock phase, the clock source phase, or the time source phase maybe determined based on the simple capture values of the two buses. Therest of the described method according to the present invention is thenexecuted in block 510.

In the case of more than two buses, it is particularly advantageousthat, when two buses are synchronized in pairs in such a manner, thejump, i.e. the correction only takes place on one of the two buses, i.e.one of the two buses assumes a master role for the global time. That is,the global time remains unchanged on the first bus, and the global timeon the second bus jumps, i.e. is changed. In this case, more than twobuses may easily be synchronized successively, using pairedsynchronization, without this synchronization of a pair having aparticularly complex influence on that of another pair.

This is to be illustrated on the basis of an example. There are fivebuses B1, B2, B3, B4, B5 in the system. The synchronizable, coupledpairs are (B1, B2), (B1, B3), (B2, B4), (B3, B5). If B1 is the masterfor B2 and B3, B2 is the master for B4, and B3 is the master for B5,then the paired synchronization (B2 and B3 are first synchronized to B1,then B4 is synchronized to B2, and B5 is synchronized to B3) results insystem-wide synchronization within two cycles.

In the same system, synchronization could also be performed without themaster principle. Then, however, the SL must ensure that the busessynchronized once also remain synchronized, i.e. that a jump or a changeon one bus also takes place on all buses synchronized with it.

In addition, it is advantageous (but not necessary) for the time masterof a bus to be synchronized to also have direct access to the globaltime of the corresponding partner bus. In this case, the SL may only beset up for the potential time masters of one bus, i.e. the SL messageregarding the level of the jump to be set is eliminated or becomes verysimple.

It is not necessary for the NTU's, i.e. the time units of the globaltime on the buses to be synchronized, to be the same. However, it isparticularly simple and useful for the (nominal) NTU's between twocoupled buses to only differ by an integral factor (powers of two in aparticularly advantageous manner).

C) Phase Adjustment by Frequency Shift

As an alternative to the mechanism described above with respect to thephase adjustment in point B), there is also the possibility of achievinga phase adjustment through a long-term change in rate (see point A inthis regard). The procedure is exactly the same in principal as thatdescribed in point A) for the frequency adjustment between two buses. Inthis case, however, the object is not to exactly adjust the NTU of thebus to the target value there, but instead to lengthen or shorten thisNTU slightly, so that the clock or timer to be adjusted is incrementedsomewhat more slowly or rapidly, and thus, a phase adjustment isachieved over a longer period of time.

D) Preserving the Synchronized State

After a phase adjustment as in B) or C), there are various options forretaining the synchronized state. On one hand, by repeating the phaseadaptation as soon as the monitored phase exceeds a particular value,and on the other hand, by adapting the frequency as described in pointA). In addition, there is a combination of these two possibilities.Since, in the method described in point A, the frequency adjustmenttypically takes place one cycle later than a corresponding change infrequency in the master bus, an accumulation of a difference may evenresult when the frequencies of the buses involved are very persuasivelymatched. In this case, a minor phase adjustment or compensation mustoccasionally be accomplished by a targeted, in particular predefinedfrequency which does not exactly match.

Use of the corresponding TTCAN interface may be demonstrated by adebugging instrument. The performance on the bus may also be analyzed bybus monitoring.

A general method of synchronization is now described below, showing howtwo or more TTCAN buses may mutually synchronize their cycle times. Hereagain, this method may be implemented by dedicated hardware, by theapplication running on the corresponding hosts, or by a special softwarelayer.

The sequence of the method, including possible variants, is describedbelow; the same prerequisites and definitions apply as was the case insynchronization of the global time, i.e. direct synchronizability whenthere is at least one gateway computer having access to both buses, andthere is a chain of gateways between two buses to be synchronized,connecting the two buses via directly synchronizable, coupled pairs.Here again, the synchronization is performed by a synchronization layerin hardware or software, which performs the synchronization and isreferred to below as synchronization layer SLZ with respect to the cycletimes. Here again, the SLZ need not necessarily be present at each node.First, the frequency adjustment between two buses will again bediscussed here in point AZ).

AZ) Frequency Adjustment between Two Buses

A frequency adjustment is only possible in TTCAN level 2 via protocolmechanisms. However, it is sufficient if the time master is operated inlevel 2 operation. This is not necessary for other nodes. There, theadjustment proceeds there like the corresponding adjustment for theglobal time, as described above, and thus cannot be made independentlyof this adjustment if the global time is also synchronized in the samenetwork.

In TTCAN level 2, the rate of the cycle time, i.e. the length of unit oftime NTUZ is determined by the frequency of the timer, in particular theoscillator of the time master and its TUR value, as already describedabove for the global time. The SLZ must ensure that the NTUZ's have thespecified ratios to one another on the different buses. In this context,the NTUZ's may be the same as or different from the NTU mentioned above.

In principle, different procedures are conceivable. Either one bus maybe adjusted to the others or two or more mutually approach each other.In addition, the adjustment may in turn take place in one step orgradually. The particular strategy depends on the SLZ and therequirements of the application. In particular, the synchronization ofthe global time and synchronization of the cycle time may be performedby the same software layer, i.e. the same synchronization layer SL,which may be expressed as follows in an advantageous embodiment:SL=SLZ.

All the methods regarding the frequency adjustment have the followingscheme in common within the scope of the cycle time:

-   -   The SLZ determines the correction to be performed for a bus and        notifies the current time master of it. The correction may be,        for example, the ratio of the instantaneous TUR value and the        new TUR value to be set.    -   It is, in turn, particularly advantageous if the SLZ determines        the correction on the time master. Then, the correction value        may be, for example, the new TUR value.    -   The SLZ determines the new TUR value in the time master and uses        it as of the following cycle.    -   All the other nodes of this bus follow the time master via TTCAN        synchronization.

This method may be explained again on the basis of FIG. 4, where thesequence is essentially the same, except that the cycle time is usedinstead of the global time. Therefore, reference is made here to thepreceding discussions of FIG. 4, and details will not be explicitlydescribed again.

ANOTHER EXAMPLE

There are two buses, B1 and B2. The synchronization strategy is that B2must adjust the length of the NTUZ to the length valid on B1. In thesimplest case, both are nominally the same length. The application (SLZ)of the current time master of B2 is assumed to have direct access to busB1, and the same oscillator is to be used for B1 and B2. Then, theNTUZ's of the two buses B1 and B2 are exactly the same length when theTUR value of this node with respect to B1 is equal to the TUR value ofthis node with respect to B2. Therefore, the SLZ must use the TUR valuewith respect to B1 as the TUR value with respect to B2.

In principle, the situation may be handled in the same way, if theNTUZ's of the two buses have any desired ratio to one another. Inhardware, it is again advantageous for the ratio or the reciprocal valueto be an integer, in particular one that is squared. If the same timer,in particular oscillator, is not used for both buses, there is again theaforementioned possibility of measuring the difference in the globaltimes of the two buses at least twice and/or periodically in succession,and calculating a correction factor from the comparison between theobserved change in the difference and the nominal change in thedifference. This possibility is also given again, if the current timemaster does not have access to both buses. As an alternative, the SLZmay measure the length of a basic cycle from one bus in units of theother bus and determine the correction value from this.

BZ) Phase Adjustment

-   -   The SLZ again measures the phase difference between two times on        two buses and determines the jump, i.e. change, to be set for        each of the two buses. The SLZ transmits the particular jump to        be set to the two current time masters of the two buses.    -   A time master having to set a jump sets, in turn, a predefined        bit, in this case, especially the Next_is_Gap bit of the TTCAN        (see ISO Draft), in the reference message. From the local SLZ,        it receives the starting time of the next basic cycle.    -   After this (these) reference message or messages is/are possibly        successfully sent on the two buses, the cycle time phases of the        two buses are synchronized.

This may again be represented by FIG. 5, as described above; the cycletime being used instead of the global time, and the Next_is_Gap bitbeing used. This desired phase may be 0 here, i.e. the basic cyclebegins at the same time on both buses. However, this is by no meansnecessary. It is not necessary for the NTUZ's, i.e. the time units ofthe cycle time, to be the same on the buses to be synchronized. However,it is particularly simple and useful if the (nominal) NTUZ's between twocoupled buses differ only by an integral factor, in particular powers oftwo, in an advantageous manner.

It is also not necessary for the cycle lengths of the buses to besynchronized to be the same. However the method proposed here isparticularly beneficial when the (nominal) cycle lengths of the twobuses are in a rational ratio to one another which does not useexcessively large natural numbers, because then it is regularly possibleto speak of a fixed phase in an easily reproducible manner. Example: Two(nominal) cycles are running on one bus B1, while three are running onanother bus B2. Then, a theoretically fixed phase relation between B1and B2 is present every other cycle on B1 (every third cycle on B2).

In the case of more than two buses, it is particularly advantageous if,with such a paired synchronization of two buses, the jump only takesplace on one of the two buses, i.e. one of the two buses assumes amaster role for the phase of the cycle time. In other words, thesequence of basic cycles on the first bus remains unchanged, while a gapis inserted between two basic cycles on the second bus, the gap beingjust large enough to adjust the desired phase. In this case, more thantwo buses may easily be synchronized successively via the pairedsynchronization without the synchronization of a pair having aparticularly complex influence on that of another pair.

EXAMPLE

There are five buses in the system, B1 through B5. Synchronizable pairsinclude (B1, B2), (B1, B3), (B2, B4), (B3, B5). If B1 is the master forB2 and B3, and B2 is the master for B4, and B3 is the master for B5,then the paired synchronization (B2 and B3 are first synchronized to B1,then B4 is synchronized to B2, and B5 is synchronized to B3) results insystem-wide synchronization within two cycles.

In the same system, synchronization could also be performed without themaster principle. Then, however, the SLZ must ensure that once buses aresynchronized, they will remain synchronized, i.e. a jump or a correctionon one bus will also take place on all buses synchronized with it.

In addition, it is advantageous (but not necessary) for the time masterof a bus to be synchronized to also have direct access to thecorresponding partner bus. In this case, the SLZ may only be set up forthe potential time masters of one bus, i.e. the SLZ message regardingthe level of the jump to be set is eliminated or becomes very simple.

CZ) Phase Adjustment by Frequency Shift

As a further alternative to the mechanism, this time the one in sectionBZ), it is also possible, here, to achieve a phase equalization bylonger-term modification of the rate, as in point AZ). The basicprocedure is exactly the same as that described in point AZ). However,the goal in this case is not to precisely adjust the NTUZ's of the busto be adjusted to the target value there, but to slightly lengthen orshorten these NTUZ's, so that the clock to be adjusted is incrementedslightly more slowly or more rapidly, and therefore, a phase adjustmentis achieved over a longer period of time.

DZ) Preserving the Synchronized State

After a phase adjustment as in BZ) or CZ), there are variouspossibilities for maintaining the synchronized state, as alreadydescribed in point D) of synchronization of the global time:

-   -   Repeating the phase adjustment as soon as the observed phase        exceeds a certain value.    -   Adjusting the frequency as in AZ) or A) or a combination of the        two possibilities.

In the method described in AZ) or A), the frequency adjustment istypically performed one cycle later than a corresponding frequencychange in the master bus, so even when there is a very precise matchingof the frequencies of the buses involved, this may result in anaccumulation of a difference. In this case, a small phase adjustment orcompensation must occasionally be performed, using a frequency whichintentionally does not match exactly. As in synchronization of theglobal time, this use may be displayed by a debugging instrument, orperformance on the bus may be analyzed with the aid of bus monitoring.

1. A method for synchronizing at least two TTCAN buses connected via atleast one bus user, comprising: determining a global time in each of theat least two TTCAN buses; determining deviations in the global times ofthe at least two TTCAN buses from the global times; transmitting thedeviations to the at least one bus user; and adjusting the global timesto one another as a function of the deviations in the global times, sothat the at least two TTCAN buses are synchronized with respect to theglobal times.
 2. The method as recited in claim 1, further comprising:adjusting cycle times of the at least two TTCAN buses by adjusting atleast one phase of the global times.
 3. The method as recited in claim1, further comprising: adjusting the global times by adjusting at leastone frequency of the global times.
 4. The method as recited in claim 2,wherein: the at least one phase is adjusted as a function of a value ofat least one predefined bit.
 5. The method as recited in claim 4,wherein: the at least one predefined bit corresponds to a discontinuitybit of TTCAN.
 6. The method as recited in claim 3, further comprising:determining, in each instance, a smallest time unit in the at least twoTTCAN buses for adjusting the at least one frequency; determining aratio of the smallest time units to produce a resulting real ratio;comparing the resulting real ratio to a predefined ratio; and adjustingat least one of the smallest time units so that the predefined ratio isattained.
 7. The method as recited in claim 6, wherein: the predefinedratio is an integer corresponding to one of a multiple of the number twoand a submultiple of the number two.
 8. A device for synchronizing atleast two TTCAN buses linked together by at least one bus user,comprising: a first arrangement for: determining a global time in eachof the at least two TTCAN buses, determining deviations in the globaltimes of the at least two TTCAN buses from the global times, andtransmitting the deviations to the at least one bus user; and a secondarrangement for adjusting the global times to one another as a functionof the deviations in the global times, so that the at least two TTCANbuses are synchronized with respect to the global times.
 9. The deviceas recited in claim 8, further comprising: a synchronization layerincluded in the at least one user and in which are contained the firstarrangement and the second arrangement.
 10. A bus system, comprising: atleast two data buses, a first data bus of the at least two data buseshaving a first number of users and a second data bus of the at least twodata buses having a second number of users, wherein: the at least twodata buses include at least two TTCAN buses, at least one user of one ofthe first number of users and the second number of users correspondingto at least one connection user, such that two of the at least two TTCANbuses are simultaneously connected to the at least one connection user,and the at least one connection user performing a time-master functionfor each TTCAN bus being included, and if the bus system includes morethan two TTCAN buses, the TTCAN buses are connected in such a mannerthat, in each instance, at least two of the TTCAN buses have at leastone common connection user; and a synchronization arrangement fordetermining a global time in each TTCAN bus and transmitting deviationsin individual global times to those of the first number of users and thesecond number of users having the time-master function for adjusting aspecific, global time.